Liquid substances applied to the airfoil surfaces of aircraft to prevent their freezing and being covered with ice in inclement weather are well-known and are important to ensure safe and proper takeoff of aircraft in winter. Deicing solutions based upon ethylene glycol and water have been used for many years to remove ice, frost and snow from aircraft surfaces, however, since they have low viscosities, they readily flow off the aircraft surfaces and thus provide limited protection from formation of additional frost. Anti-icing fluids are those that will prevent formation of frost and ice on surfaces over extended periods such as overnight and in the case of delays between departure from the gate and takeoff.
The fluids may be diluted somewhat through intentional dilution with water, such as during particularly cold conditions below the expected operating temperatures of the fluids to enable their application, or incidentally as through freezing rain conditions. In addition to these demands, the deicing materials must also exhibit Newtonian rheology, that is, a viscosity that is shear-independent and time-independent. They simply flow off the wing surfaces after deicing. Such compositions are also useful in the deicing of other surfaces besides air foils, for example, runway and vehicle roadway surfaces.
It is also well-known that aircraft departures are often delayed under inclement weather conditions and the anti-icing formulations must often be reapplied if the plane must wait an extended period. These liquids must be stable not only through temperature extremes, but also during the taxiing phases of the takeoff procedure and thus must adhere to the wing surfaces during travel and ground winds.
Deicing/anti-icing can be performed in one or two steps. One-step deicing/anti-icing is carried out with an anti-icing fluid. The fluid is used to deice the aircraft and remains on aircraft surfaces to provide limited anti-ice capability. Two-step deicing/anti-icing consists of two distinct steps. The first step (deicing) is followed by the second step (anti-icing) as a separate fluid application. Anti-icing fluid is applied to protect the relevant surfaces thus providing maximum possible anti-ice capability. However, usually two different trucks and attendant systems are used to apply the deicing fluid and then the anti-icing fluid. It takes a few minutes to move the first deicing truck away and the second, anti-icing truck, into position. Holdover times are thus important for deicing fluids because the deicing fluid should remain on the wing to protect it until the anti-icing fluid is applied. If the deicing fluid does not stay on the surface during this period, under some weather conditions clear ice can form in the interim which is quite undesirable. For more information, see SAE Aerospace Recommended Practice: Issued Proposed Draft (SAE ARP4737) (SAE-i.e. Society of Automotive Engineers, Inc.).
Two main categories of deicing/anti-icing fluids have developed, Type I and Type II. It is important to keep in mind the difference between aircraft deicing, which is the procedure by which frost, ice or snow is removed from the aircraft in order to provide clean surfaces, and aircraft anti-icing, which is the procedure which provides protection against the formation of frost or ice and accumulation of snow or slush on clean surfaces of the aircraft for a limited period of time. While there are similarities between deicing compositions and anti-icing compositions, there are some important differences, too. As mentioned, deicing compositions have Newtonian rheology. The shear rate of a Newtonian fluid is directly proportional to the shear stress. The fluid will begin to move immediately upon application of a stress; it has no yield stress which must be accomplished before flow begins. One publication describing the important differences between deicing compositions and anti-icing compositions (including those mentioned above) is M. S. Jarrell, "SAE Type I Aircraft Deicing/Anti-icing Fluids," FAA International Conference on Airplane Ground Deicing, May 28-29, 1992, Reston, Va.
Type I is also described in "Deicing/Anti-Icing Fluid, Aircraft, Newtonian-SAE Type I"; Aerospace Material Specification, AMS 1424, Jan. 1, 1993. (Available from SAE International, 400 Commonwealth Drive, Warrendale, Pa. 15096-0001). This publication sets forth technical requirements, quality assurance provisions, etc. and is submitted for recognition as an American National Standard.
Type I is characterized as unthickened, is at least 80% glycol, the viscosity depends on the temperature and it demonstrates limited anti-icing capability relative to holdover time. Hill, E. G., et al., "Effects of Deicing Fluids", AIRLINER, October-December 1989.
Another publication addresses non-Newtonian, pseudo plastic fluids, designated SAE Type II. "Fluid, Aircraft Deicing/Anti-Icing, Non-Newtonian, Pseudo-Plastic, SAE Type II", Aerospace Material Specification, AMS 1428, Jan. 1, 1993.
Type II is characterized as thickened, is at least 50% glycol, the viscosity depends on temperature and shear rate acting on the fluid and it demonstrates good anti-icing capability, Hill, supra, p. 2.
Various deicing and anti-icing compositions are known. For example, U.S. Pat. No. 4,358,389, to Konig-Lumer et al. (1982) discloses a liquid agent for deicing which is essentially composed of (a) glycols, primarily propylene glycol, (b) water, (c) thickeners, (d) substances insoluble in water, (e) surface-active agents, (f) corrosion inhibitors and alkaline compounds, having a pH value of 7.5 to 10. The object of this invention was to find an agent which fulfilled important demands such as stability against shear, viscosity, rheological behavior and holdover time. This research resulted in a thickener comprising cross-linked polyacrylates having specific physical parameters.
U.S. Pat. No. 4,954,279, to Ma et al. (1990), discloses another approach to anti-icing, deicing compositions comprising the use of a continuous phase and a discontinuous phase, wherein the continuous phase contains the glycol (primarily ethylene glycol) and water and a discontinuous phase contains a water-insoluble oil and the oil comprises at least one substantially water-insoluble partially polar compound, such as, for example, carboxylic, sulfonic and phosphonic acids, salts or polar esters having hydrocarbyl substituents of at least about 6 carbon atoms per group.
U.S. Pat. No. 4,744,913, to Salvador et al. (1988) discloses a deicing and anti-icing agent based on glycols and water (primarily diethylene glycol and propylene glycol) and containing cross-linked acrylic polymers as the thickener. The composition also contains customary corrosion inhibitors, surfactants belonging to the group of alkali metal alkylarylsulfonates and a neutralizing agent to adjust the pH to a basic value. The thickener comprises two cross-linked acrylic polymers in a specific ratio by weight to one another, namely a selected cross-linked acrylic acid or alkali metal acrylate homopolymer and a selected cross-linked acrylic acid/acrylamide or alkali metal acrylate/acrylamide copolymer in a ratio by weight of 2:1 to 10:1.
U.S. Pat. No. 4,698,172, to Tye et al. (1987) discloses the use of carrageenan, a sulfated polysaccharide gum derived from marine algae, in a glycol-based anti-icing fluid (preferably ethylene glycol) in an amount sufficient to thicken the fluid to promote its adherence to aircraft surfaces (less than 5 wt %, preferably 0.05 to 3 wt % and more preferably 0.1 to 1 wt %). It is stated the fluid thickened by this substance does not adversely affect airfoil lift characteristics during take-off because the fluid exhibits shear thinning.
U.S. Pat. No. 4,585,571, to Bloom (1986) discloses a deicing fluid which is claimed to effectively remove existing ice and prevent its reformation for at least eight hours without adversely affecting the compatibility properties of the fluid. The deicing fluid contains, in addition to an alkylene polyol, an anionic surfactant capable of forming a hydrophobic monolayer on the metal surfaces of the aircraft, a hydrophilic wetting agent which is capable of associating with the hydrophobic monolayer, and a coupling agent which facilitates the association between the wetting agent and monolayer.
Chemical Abstract 108:223486p (1988) of Romanian patent document RO 92,551 briefly sets out sprayable deicing compositions that form films with high shear resistance on aircraft which contain 250-500 parts propylene glycol, 5-15 parts polyacrylamide, 0.5-1 part Na.sub.2 HPO.sub.4 or borax, 0.5-1 part ethoxylated nonylphenol or oleyl alcohol with a degree of ethoxylation of 17, and 250-500 parts of water.
British Patent Specification 1,272,464 discusses a deicing fluid having an aqueous solution of an alcohol component which comprises one or more of ethylene glycol, propylene glycol and glycerol; a polyethylene glycol ether of a diisoalkylphenol; and a thickening agent which is a copolymer of acrylic acid or methacrylic acid.
An antifreeze mixture for internal combustion engines based on ethylene glycol and/or propylene glycol which is free of nitrates and phosphates is disclosed in U.S. Pat. No. 5,064,552, to Oppenlaender et al. (1991).
U.S. Pat. No. 5,118,434, to Meyer et al. (1992) addresses the problem of precipitation of salts from glycol-based fluids and discloses a method for adding copolymers or terpolymers of acrylic acid or the alkali metal salt thereof and one or more monomers selected from the group consisting of 2-acrylamido-2-methylpropyl sulfonic acid or metal salt thereof, a lower alkyl hydroxy acrylate, and mixtures thereof in order to prevent the precipitation. The glycol-based fluids are mentioned as deicing fluids.
It is noted that most of the deicing/anti-icing fluids discussed in the art which contain high molecular weight acrylic acid thickeners produce non-Newtonian or Type II rheology.
It is also noted that in the past, when those in the art attempted new formulations, whether Type I or Type II, they typically encountered additional problems with hard water compatibility, thermal stability and corrosion protection properties.
Further, it does not appear the criticality of wetting agents has been addressed in the art. The identification of wetting agents which maintain stability at higher temperatures and pH appears to be a critical step in formulating a composition with the improved properties claimed herein.
Generally deicing fluids known in the art have not exhibited long lasting protection against ice reformation without compromising the compatibility properties of the deicing fluid. There would be a great commercial potential in the art for a deicing fluid which exhibits extended anti-icing holdover time, but which, unlike prior art high molecular weight acrylic acid thickened fluids which produce non-Newtonian rheology, would still have Type I, Newtonian rheology and could pass the requirements for a Type I, Newtonian type fluid.
It would also be valuable if a formula were available which possessed both deicing and anti-icing properties and also provided improvements in hard water compatibility, thermal stability and corrosion protection.